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Climate change is mainly caused by greenhouse gases, such as carbon dioxide. And the gases mainly come from sources like power plants and vehicle emissions. But scientists have found another major threat to the climate: black carbon particles from wood fires, diesel engines and practically everything else that’s burned.

Carbon dioxide has long been the villain behind climate change. That’s still the case, but new research suggests CO2 has a sidekick—black carbon.

That’s not just a clever nickname. Black carbon is pretty much exactly what it sounds like. Carbon particles that are black in color. In the United States, most black carbon comes from diesel engines, but wood burning and cook stoves in other countries also create a lot of it.

To understand black carbon’s role in climate change, you need to think about it two different ways: on snow and in the air. We’ll start with snow.

‘A Feedback Cycle’

In the Louisville Zoo earlier this week, Qannik the polar bear cub grunted while her trainer fed her chunks of meat. Qannik came to the zoo in 2011, after she was separated from her mother. Scientists suspect she’s a casualty of climate change, a story that’s becoming increasingly common in the Arctic as snow and sea ice melt and the polar bear’s hunting season shortens.

One of the reasons snow and ice are melting in the Arctic is because carbon dioxide in the atmosphere is causing the climate to warm. But another major one is black carbon, for a pretty obvious reason:

When black carbon falls on snow and it absorbs more sunlight, it melts sooner, Doherty said.

“And if it melts sooner, the snow and ice itself is fairly white, but when it melts it exposes the underlying surface, which is either ocean or land, and that’s quite a bit darker than the snowpack,” she said. “So that absorbs more sunlight. So there’s sort of a feedback cycle where if you make the snow melt earlier, you get even more warming so it melts even more quickly.”

That’s one of the reasons black carbon is also harmful to the atmosphere—simply because of its color.

Another of the study’s authors—Tami Bond of the University of Illinois—said when black carbon gets into the atmosphere, it ends up in clouds. These clouds usually reflect the sunlight and help cool the planet, but something else happens when they contain more and more black carbon.

“I think that everybody has had the experience of walking across a black parking lot on a hot day,” Bond said. “The asphalt absorbs light and puts it into the atmosphere, and that’s why you feel hot. And that’s what black carbon is doing as it’s floating in the atmosphere.”

Black Particles vs. Light Particles

But—like everything else with air pollution—black carbon’s effect on the climate is more complicated than it first appears.

Most of the time, black carbon isn’t emitted by itself. It enters the air with numerous other particles, as well as greenhouse gases.

Ignoring the greenhouse gases for a moment—they’ve been shown to have a warming effect on the climate—the particle pollution emitted is often a mixed bag. A lot of the particles that are emitted actually cool climate, because they’re lighter. During a forest fire, the particulates themselves have a net cooling effect. Which is why it’s important to know the ratio of black carbon to lighter-colored particles during combustion.

But scientists know the ratio for diesel exhaust. It has a high concentration of black carbon, and the particles have a net warming effect on the planet.

“If you were definitely trying to target sources that reducing their particulate emissions would slow climate warming, diesel emissions would be a good one to go after,” Doherty said.

Immediate Results

Black carbon can travel pretty far, and only lasts for anywhere from a week to three weeks. But scientists estimate that about three-quarters of the black carbon within a mile above a city is from local emissions. Higher up, you’ll find black carbon from cities upwind. This means a lot of the black carbon emitted in a city like Louisville stays here, and contributes to warming here.

Bond says black carbon’s short life means there’s an opportunity to reduce emissions and see an immediate change.

“The heating is reduced as soon as you stop emissions,” she said. “The second thing is that nobody really wants black carbon, because particles all have health effects, so people have a vested interest in reducing particulate matter, or particles, anyway.”

So reducing black carbon emissions can slow climate change. But it won’t stop it. Climate scientists estimate that the post-industrial world is on track to burn its trillionth metric ton of carbon by 2040, and it’ll take long-term action, like cutting down on greenhouse gases, to solve the problem beyond tomorrow.